Most solenoid actuators have a ferromagnetic casing. The casing encircles a coil which is typically wrapped in a polymeric bobbin. Within the coil is a core assembly or core. An armature (or armature assembly), slides within the core (or core assembly). The armature is moved via flux transfer as current is run through the coil. The flux loop is completed by flux transfer from the casing to the core to the armature and back through the core to the casing. Armature translation is accomplished by transfer of flux from the upper section of the core (sometimes referred to as the flux return or flux tube) through the armature to the lower section of the core (shunt end or shunt). For this reason, there is a thin section of the core in between the upper and lower sections (commonly referred to as the flux choke) to reduce the amount of flux directly transferring from the upper to lower core.
While flux is intended to travel from the upper to lower core through the armature, it is desirable for this flux path to be more axially oriented than radially oriented in the lower section of the core (shunt end). Radial flux transfer in this portion of the assembly leads to higher magnetic side loading of the armature and thus higher frictional forces. It is desirable that the armature be designed to reduce the relative amount of side loading of the armature within the core.
In order to guide the armature through the core and minimize armature misalignment, it is desirable to maximize the bearing length. With a continuous steel armature, as the bearing length is increased, the lower section of the bearing goes deeper into the shunt, causing higher side loading. Even when the steel armature or core bearing surface is plated or coated in some way, side loads remain high. It is desirable to decouple the lower bearing from the magnetic portion of the armature.